LeetCode - Read N Characters Given Read4 II - Call multiple times

The API: int read4(char *buf) reads 4 characters at a time from a file.

The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file.

By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.

Note:
The read function may be called multiple times.

// Forward declaration of the read4 API.int read4(char *buf);class Solution {public:    /**     * @param buf Destination buffer     * @param n   Maximum number of characters to read     * @return    The number of characters read     */    int read(char *buf, int n) {        int cnt = 0;        int sz;        if(buf_len > 0){            int mn = min(buf_len, n);            memcpy(buf, buffer, mn);            cnt += mn;            if(n < buf_len){                memcpy(buffer, buffer + n, buf_len - n);            }            buf_len -= mn;        }        int leftsz = 0;        while(cnt < n){            sz = read4(buffer);            memcpy(buf + cnt, buffer, min(n - cnt, sz));            leftsz = sz - min(n - cnt, sz);            cnt += min(n - cnt, sz);            if(sz < 4) break;        }        if(leftsz != 0){            buf_len = leftsz;            memcpy(buffer, buffer + sz - leftsz, leftsz);        }        return cnt;    }    char buffer[4];    int buf_len = 0;};